Method for damping relative movements occurring in a work vehicle during advance

ABSTRACT

A method for damping relative movements occurring between a first and a second part of a work vehicle during advance of the vehicle, which parts are interconnected by means of at least one hydraulic actuator, includes detecting at least one operation parameter of the vehicle, determining whether a damping condition is present based on the detected operation parameter and if the determined damping condition is present, controlling a displacement of a variable displacement hydraulic motor arranged downstream of the actuator in such a way that energy from a hydraulic fluid transmitted from the actuator is converted to a rotational energy in the motor, and transmitting the recovered energy from the motor to a power source.

BACKGROUND AND SUMMARY

The present invention relates to a method for damping relative movementsoccurring between a first and a second part of a work vehicle duringadvance of the vehicle, which parts are interconnected by means of atleast one hydraulic actuator.

The term work vehicle comprises different types of material handlingvehicles like construction machines, such as a wheel loader, a backhoeloader, a motor grader and an excavator. The invention will be describedbelow in a case in which it is applied in a wheel loader. This is to beregarded only as an example of a preferred application.

Said actuator may be a linear actuator in the form of a hydrauliccylinder. A wheel loader comprises several such hydraulic cylinders inorder to perform certain work functions. A first pair of hydrauliccylinders is arranged for turning (steering) the wheel loader. A secondpair of hydraulic cylinders is arranged for lifting load-arm unit and afurther hydraulic cylinder is arranged on the load arm unit for tiltingan implement, for example a bucket or forks, arranged on the load armunit.

It is known to damp relative movements between a load-arm unit and aforward vehicle section in a wheel loader by means of a load-arm unitsuspension system.

By damping the movements, the comfort of the driver in the machine isincreased and material that is being carried by the implement isprevented from leaving the implement. If, for example, a bucket isarranged on the load-arm unit, it is desirable that the material that isloaded in the bucket does not fall out of the bucket when the vehiclegoes over a bump. A vehicle provided with large tyres uses the tyres assprings during advancing over an uneven surface. However, the tyres arenot capable of effectively damping the jumping movements and pitchingoscillations that occur in the vehicle body when the vehicle travels onan uneven surface.

When the vehicle goes over a bump in the surface, the vehicle body movesupwards. On account of the mass inertia in the load-arm unit, theload-arm unit tends to stay at its existing level above the surface.Instead of the load-arm unit following the vehicle body upwards, thepistons of the cylinders are forced into the cylinders, which means thathydraulic oil flows out of the cylinders.

According to a known suspension system, see for example WO 99/16981, anaccumulator is arranged downstream of the cylinders. The gas present inthe accumulator will thus be compressed when the vehicle goes over abump. The pistons will be displaced into the cylinders as long as thepressure in the cylinders is lower than the pressure that is needed inorder to overcome the accelerating force and the force of gravity fromthe load assembly. When the machine goes over a hole in the surface, thereverse sequence occurs, that is to say that hydraulic oil flows fromthe accumulator to the cylinders.

During work with the machine in, for example, a gravel pit, the load armsuspension system should be deactivated when the bucket is to be filled.The vehicle then drives with great force into a gravel heap, with thebucket located in front of it. It is then desirable that the load-armunit is rotationally rigid and that the pistons in the cylindersmaintain their set position. Subsequently, when the machine is totransport the gravel in the bucket, the load-arm unit suspension systemis activated. On activation of the load arm suspension system, the loadarm assembly is to maintain its set position.

The known suspension system in WO 99/16981 is passive. The liftcylinders are connected to at least one gas filled accumulator, whichfunctions as a spring. The hydraulic fluid flowing with a certainpressure drop back and forth between the cylinder and the accumulatorfunctions as a dampener. Further, friction in the load-arm unit and thecylinder contribute to the dampening function.

Further, when traveling over an uneven ground, large side accelerationforces are created around a steering joint in a frame steered vehicle,like a wheel loader. It is known to use accumulators in fluid connectionto the steering cylinders, which accumulators have the effect of dampingthe relative movements between the frame parts also when the steeringcylinders are not activated for steering the vehicle.

It is desirable to achieve a method for dampening the relative movementsbetween a first and a second part of the work vehicle during advance ofthe vehicle, which recovers energy from said relative movements.

The method according to an aspect of the present invention comprises thesteps of detecting at least one operation parameter of the vehicle,determining whether a damping condition is present based on the detectedoperation parameter and if the determined damping condition is present:controlling a displacement of a variable displacement hydraulic motorarranged downstream of the actuator in such a way that energy from ahydraulic fluid transmitted from the actuator is converted to arotational energy in the motor, and transmitting the recovered energyfrom the motor to a power source.

Thus, the kinetic energy from undesired relative movements of thevehicle, like rocking or oscillating movements, will be recovered bythis method. Further, by virtue of this energy recovery method, fuelconsumption is reduced.

The method may be used for damping and energy recovery from severalfunctions of the vehicle, like from the lifting function, tiltingfunction and steering function. Relative motions may be damped andenergy may be recovered simultaneously with a suitably designed system.

According to an aspect of the invention, the detected operationparameter is an angle between the first and second parts and/or ahydraulic fluid pressure associated to the actuator.

For example, the damping condition may be determined based on apredetermined change in said detected hydraulic fluid pressure. Further,one may determine if the desired pressure change is present forinitiating an energy recovery phase by determining the frequency contentof the detected pressure and/or a derivative of the detected pressure.In this way, it is determined if there is an operation conditionpresent, such as a permanent rise in the pressure due to a heavier loador a temporary disturbance, which will not initiate an energy recoveryphase.

According to an aspect of the invention, the method comprises the stepof automatically moving the first and second parts relative to oneanother to a specific relative position, preferably by means of saidactuator, after termination of an energy recovery phase. Said specificrelative position is preferably the initial relative position of the twoparts prior to initiation of the energy recovery operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained below, with reference to the embodimentsshown on the appended drawings, wherein

FIG. 1 schematically shows a wheel loader in a side view,

FIG. 2 shows a system for energy recovery of a lifting function duringoperation of the wheel loader,

FIGS. 3 and 4 show an alternative system to the one shown in FIG. 2,

FIGS. 5 and 6 show a system for energy recovery of a tilting functionduring operation of the wheel loader,

FIGS. 7 and 8 show a system for energy recovery of a steering functionduring operation of the wheel loader, and

FIG. 9 shows a system for energy recovery in a wheel loader comprising asteering, lifting and tilting function.

DETAILED DESCRIPTION

FIG. 1 shows a wheel loader 1. The wheel loader 1 comprises a forwardsection 2 and a rear section 3, which sections each has a pair of halfshafts 12, 13. The forward vehicle section 2 comprises a first framepart and the rear vehicle section 3 comprises a second frame part. Therear section 3 comprises a cab 14. The vehicle sections 2, 3 areconnected to each other in such a way that they can pivot in relation toeach other around a vertical axis by means of two actuators in the formof hydraulic cylinders 4,5 arranged between the two sections. Thehydraulic cylinders 4,5 are thus arranged one on each side of ahorizontal centerline of the vehicle in a vehicle traveling direction inorder to turn the wheel loader 1.

The wheel loader 1 comprises an equipment 11 for handling objects ormaterial. The equipment 11 comprises a load-arm unit 6 and an implement7 in the form of a bucket fitted on the load-arm unit. A first end ofthe load-arm unit 6 is pivotally connected to the forward vehiclesection 2. The implement 7 is pivotally connected to a second end of theload-arm unit 6.

The load-arm unit 6 can be raised and lowered relative to the forwardsection 2 of the vehicle by means of two second actuators in the form oftwo hydraulic cylinders 8,9, each of which is connected at one end tothe forward vehicle section 2 and at the other end to the load-arm unit6. The bucket 7 can be tilted relative to the load-arm unit 6 by meansof a third actuator in the form of a hydraulic cylinder 10, which isconnected at one end to the forward vehicle section 2 and at the otherend to the bucket 7 via a link-arm system 15.

FIG. 2 shows a system for damping relative movements and energy recoveryin a hydraulic circuit 100 arranged for a lifting operation of theload-arm unit 6. The two lifting cylinders 8,9 from the arrangementshown in FIG. 1 are here, for ease of presentation, replaced by onehydraulic cylinder 101. The arrangement comprises a power source 103 inthe form of a diesel engine for propelling the wheel loader. Thearrangement further comprises a pump 104, which is rotatably driven bythe power source 103.

The hydraulic cylinder 101 is arranged in fluid connection with the pump104 via a first conduit 105. A variable displacement hydraulic motorunit 106 is arranged in fluid connection with the cylinder 101 anddownstream the cylinder via a second conduit 107. Said motor unit 106comprises a single motor. A fluid container 120 is arranged downstreamof the motor 106 for collecting fluid.

The first conduit 105 is branched off in two input conduits 108, 109 tothe cylinder. A first input conduit 108 is connected to a piston sideand a second input conduit 109 is connected to a piston rod side. Twooutput conduits 110, 111 are also connected to the cylinder. A firstoutput conduit 110 is connected to the piston rod side and a secondoutput conduit 111 is connected to the piston side. The two outputconduits 110, 111 merges to the second conduit 107.

An on/off valve 112, 113, 114, 115 is arranged on each of these fourinput/output conduits 108, 109, 110, 111. By simultaneously open theon/off valve 112 on the first input conduit 108 and the on/off valve 114on the first output conduit 110, the load-arm unit 6 may be raised. Inthe same way, by simultaneously open the on/off valve 113 on the secondinput conduit 109 and the on/off valve 115 on the second output conduit111, the load-arm unit 6 may be lowered.

The system comprises a controller, or electronic control unit, 116,which is connected to each of the on/off valves 112, 113, 114, 115 forelectrically controlling them, see dashed lines.

The system further comprises a control lever, or joystick, 117 foroperation by an operator. The control lever 117 is electricallyconnected to the controller 116. Operation of the control lever 117generates a work function signal indicative of a requested raising orlowering of the load-arm unit 6.

All hydraulic fluid from the hydraulic cylinder 101 passes the motor106. By varying the displacement of the motor 106, the movement speed ofthe cylinder 101 is controlled during a lifting/lowering operation.Thus, the system in FIG. 2 may not only be arranged for damping andenergy recovery, but also be used for moving the cylinder 101.

The system comprises means 118 for determining an angle between an arm119 of the load-arm unit 6 and the forward vehicle section 2. Saiddetection means 118 is electrically coupled to the controller 116 and isarranged to sense the position of the arm 119. As an alternative, theangle determining means may be formed by an angular sensor arranged atthe joint between the load-arm unit 6 and the forward vehicle section 2.As a further alternative, the angle determining means 118 may be formedby a sensor arranged to sense the extension of the lifting cylinder 101.

Means 121, 122 are provided for sensing a load pressure subjected to thecylinder 101 during operation. Said sensing means is formed byelectrical pressure sensors 121, 122, which generate pressure signals tothe controller 116. A first pressure sensor 121 is arranged on theoutput conduit 110 at the piston rod side of the cylinder 101 and asecond pressure sensor 122 is arranged on the output conduit 111 at thepiston side of the cylinder 101.

The variable displacement hydraulic motor 106 controls the speed of themovement of the load-arm unit 6 during normal operation, i.e. when thereis no damping. Further, the fluid connection through the first conduit105, 108, 109 from the pump 104 to the cylinder 101 is free fromactuator movement controlling throttling means. The controller 116 iselectrically connected to the motor 106 for adjusting the displacement.

The diesel engine 103 mechanically drives the pump 104 via a drive shaft150. The drive shaft 150 is also mechanically connected to the motor106. Thus, the pump 104 and the motor 106 rotates at the same speedduring operation. An alternative embodiment will be described below withreference to FIG. 9.

A method for damping relative movements between the forward vehiclesection 2 and the load-arm unit 6 and the energy recovery from therelative movement will be described below.

During transport of the wheel loader 1, the on/off valves 112, 113, 114,115 are closed. When the wheel loader 1 travels over a bump in theground, the forward vehicle section 2 will move upwards. Morespecifically, the forward vehicle section 2 will move upwards relativeto the load-arm unit 6. Said upward movement of the forward vehiclesection 2 leads to an increased pressure on the output piston side ofthe cylinder 101, which is detected by means of the second pressuresensor 122. The controller 116 continuously receives and registersinformation of the pressure from the pressure sensors 121, 122. Thus,the controller 116 has information about a ground pressure in thecylinder 116 before the vehicle reaches the bump.

When the detected pressure exceeds the ground pressure to a certainextent, the controller automatically opens the associated on/off valve115 on the output piston side of the cylinder 101. Thereby, a fluidconnection from the actuator 101 to the motor 106 is opened. Further,the displacement of the variable displacement hydraulic motor 106 iscontrolled in such a manner that energy from the hydraulic fluid flow istransmitted from the motor 106 to the engine 103. The controller 116also opens the on/off valve 113 on the input piston rod side of thecylinder 101 so that the pump 104 supplies pressurized hydraulic fluidto the input piston rod side of the cylinder 101 during the energyrecovery phase. According to an alternative to using the pump, hydraulicfluid may be supplied to the piston rod side of the cylinder 101 via anafter fill valve unit, see for example reference numeral 128 in FIG. 3.

A termination of an energy recovery phase is determined by repeatedlydetecting at least one of said operation parameters of the vehicle, i.ethe pressure and/or angle values. After said termination of the energyrecovery phase, the system is controlled so that the load-arm unit 6resumes its prior position relative to the forward vehicle section 2after termination of the energy recovery phase. The on/off valve 113 onthe input piston side of the cylinder 101 is closed and the on/off valve112 on the input piston side of the cylinder 101 is opened.

The pump 104 will continue to supply pressurized hydraulic fluid to theinput piston side of the cylinder 101 and the on/off valve 114 on theoutput piston rod side will be opened so that the load-arm unit 6resumes said prior position.

According to an alternative or complementary step, said upward movementof the forward vehicle section 2 is detected by means of the angledetermining means 118. The controller 116 continuously receives andregisters information of the angle from the angle determining means 118.Thus, the controller 116 has information about an initial relative anglebetween the load-arm unit 6 and the forward vehicle section 2 before thevehicle reaches the bump.

A downward movement of the forward vehicle section 2, due to a recess inthe ground, is likewise determined by means of the pressure sensors 121,122 and/or the angle determining means 118. The controller 116 receivesinformation of the determined downward movement and in responseautomatically opens the on/off valve 114 on the output piston rod sideof the cylinder 101. A fluid connection from the actuator 101 to themotor 106 is then opened. Further, the displacement of the variabledisplacement hydraulic motor 106 is controlled in such a manner thatenergy is transmitted from the motor 106 to the engine 103. Further, theon/off valve 112 is opened so that the pump 104 supplies pressurizedhydraulic fluid to the input piston side of the cylinder 101 during theenergy recovery phase.

After termination of the energy recovery phase, the controller 116closes the on/off valve 112 on the input piston side and opens theon/off valve 113 on the input piston rod side of the cylinder 101.Further, the on/off valve 114 on the piston rod side is closed and theon/off valve 115 on the piston side is opened. The pump 104 will thensupply pressurized hydraulic fluid to the input piston rod side of thecylinder 101 so that the load-arm unit 6 resumes its prior positionrelative to the forward vehicle section 2.

Thus, in principle, when the detected pressure on either side of thecylinder 101 exceeds the ground pressure to a predetermined extent, theoutlet on/off valve on that side is automatically opened so that theenergy can be recovered via the motor 106. The angular position of theload-arm unit 6 relative to the forward vehicle section 2 is thereafteradjusted to its initial position before the vehicle reached theunevenness in the ground by supplying pressurized fluid to the said sideof the cylinder 101.

The limit of the magnitude of the movement of the load-arm unit 6relative to the forward vehicle section 2 for initiating energy recoveryis determined as a function of the detected pressure increase.

Thus, the motor 106 has two functions: 1) recovering energy to theengine 103 during the energy recovery phase during a transportoperation, and 2) controlling the movement speed of the cylinder 101during a lifting/lowering of the load-arm unit 6.

The recovered kinetic energy from the downward movement of the forwardvehicle section 2 is larger than the energy consumed by the pump 104during the travel over the bump/recess, which gives a positive netresult.

By virtue of the fact that the kinetic energy from the downward movementof the forward vehicle section 2 is converted to rotational energyrecovered to the power source 103, the oscillating movements of thevehicle will be dampened.

An alternative embodiment of the hydraulic system 100 of FIG. 2 is shownin FIGS. 3 and 4. The hydraulic system 201 in FIGS. 3 and 4 differs fromthe system 100 in FIG. 1 in the following respects:

A first bypass conduit 123 is connected to the first output conduit 110from the piston rod side of the cylinder 101 upstream of the associatedon/off valve 114 and to a conduit 124 downstream of the motor 106. Anonreturn valve 125 is arranged on the first bypass conduit 123. Asecond bypass conduit 126 is connected to the second output conduit 111from the piston, side of the cylinder 101 upstream of the associatedon/off valve 115 and to the conduit 124 downstream of the motor 106. Anon-return valve 127 is arranged on the second bypass conduit 126.

A valve unit 128 is arranged on the conduit 124 between the motor 106and the hydraulic fluid container 120. The valve unit 128 is arranged toclose and open, respectively the fluid connection from the motor 106 tothe tank 120. The valve unit 128 is further arranged downstream of thejoint of the bypass conduits 123, 126 and the conduit 124 from the motor106. The valve unit 128 comprises a spring loaded valve 129 on theconduit 124. The valve 129 is controlled by the pressure in the conduit128 upstream of the valve 129 via a pilot conduit 130.

FIGS. 3 and 4 show the same system 201 in two different states of themethod when the vehicle reaches a raised portion in the ground. Themethod steps performed are described when the vehicle reaches a raisedportion in the ground.

FIG. 3 shows the positions of the on/off valves during the energyrecovery phase described above with reference to FIG. 2. An arrow 132 inFIG. 3 illustrates that the vehicle has reached a bump in the groundwherein the cylinder 101 is compressed. FIG. 4 shows the positions ofthe on/off valves after termination of the energy recovery phase whenthe load-arm unit 6 is automatically returned to its prior positionrelative to the forward vehicle section 2, see arrow 131 in FIG. 4 andthe description above with reference to FIG. 2.

The hydraulic system 201 shown in FIGS. 3 and 4 may also be used in analternative method. According to this alternative method, the pump 104will not supply pressurized fluid to the cylinder 101 during the energyrecovery phase. Instead hydraulic energy from undesired relativemovements between the load-arm unit 6 and the forward vehicle section 2will be used to after fill the cylinder 101. Only the main differencesof this alternative method relative to the method above will bedescribed below.

The downward arrow 132 in FIG. 3 illustrates that the forward vehiclesection 2 moves upwards relative to the load-arm unit 6. The liftcylinder 101 will then be compressed. However, when the controllerreceives information of the relative motion and an associated pressureincrease on the output piston side, it will not open the on/off valve115 on the output piston side of the cylinder 101. The load-arm unit 6will then pitch and the pressure will next increase on the piston rodside instead. The on/off valve 114 on the output piston rod side willthen automatically be opened by the controller. Thereby, a fluidconnection from the cylinder 101 to the motor 106 is opened and thedisplacement of the variable displacement hydraulic motor 106 iscontrolled in such a manner that energy is transmitted from the motor106 to the engine 103. The further on/off valves 112, 113, 115 remainclosed.

Further, the valve unit 128 will be in a position in which the fluidconnection between the motor 106 and the tank 120 is closed. The fluidfrom the motor 106 will then be guided to the piston side of thecylinder 101 via the second bypass conduit 126 and after fill thecylinder 101. Note that since the pressure in the conduit on the outputpiston rod side of the cylinder 101 is high, the non-return valve 125 onthe first bypass conduit 123 will be closed.

The on/off valve 114 on the piston rod side is closed when the pressureon the piston rod side is decreased to a certain extent or when themovement of the load-arm unit 6 is decreased to a certain extent. Theon/off valve 115 on the piston side is instead opened so that theload-arm unit 6 is moved downwards relative to the forward vehiclesection 2 and the energy is recovered via the motor 106. When theload-arm unit 6 reaches its prior, initial position, the on/off valve115 on the piston side is closed. The method may now be repeated again.

A similar method as the alternative method described above is used whenthe vehicle 1 instead reaches a recessed portion in the ground. Theforward vehicle section 2 will then move downwards relative to theload-arm unit 6. This relative movement is likewise determined by meansof the pressure sensors 121, 122 and/or the angle determining means 118.The controller receives information of the determined downward movementand in response automatically opens the on/off valve 114 on the outputpiston rod side 101. A fluid connection from the actuator 101 to themotor 106 is then opened and the displacement of the variabledisplacement hydraulic motor 106 is controlled in such a manner thatenergy is transmitted from the motor 106 to the engine 103. The furthermethod steps are the same as has been described above for thealternative method when the vehicle reaches a bump after the pitchingand energy recovery via the on/off valve 114 on the piston rod side.

According to the alternative method described above, a sufficient afterfilling is required. Therefore, according to a variant, an accumulator(not shown) may be connected to the by-pass conduits 123, 126. Accordingto a further complement/alternative, the pump 104 may be connectedduring the after filling in order to add further fluid flow ifnecessary. The controller can determine if such after filling isnecessary based on the detected pressure on the piston side. Accordingto a further variant, the pump 104 is connected to the bypass conduits123, 126 and adds a low pressure to the after filling output side of thecylinder.

FIGS. 5 and 6 shows a hydraulic system 202 arranged for a tiltingoperation of the implement 7 relative to the load-arm unit 6. The systemand energy recovery method for the tilting function is very similar towhat has been described above for the lifting function. When the vehicle1 reaches a raised portion in the ground, the load-arm unit 6 will moverelative to the implement 7 by a clockwise rotating motion due to themass inertia in the implement 7, see arrow 132 in FIG. 5. The tiltpiston of the tilt cylinder 10 is then forced into the cylinder.

When the vehicle 1 instead reaches a recessed portion in the ground, theload-arm unit 6 will move relative to the implement 7 by acounterclockwise rotating motion due to the mass inertia in theimplement 7.

In the same way as has been described above for the alternative liftinghydraulic circuit 201, the power source 103 in the form of a dieselengine for propelling the wheel loader rotatably drives the pump 104.The tilt cylinder 10 is arranged in fluid connection with the pump 104via a first conduit. A variable displacement hydraulic motor unit 133 isarranged in fluid connection with the cylinder 10 and downstream thetilt cylinder 10 via a second conduit. Said motor unit 133 comprises asingle motor. The fluid container 120 is arranged downstream of themotor 133 for collecting hydraulic fluid.

The arrangement of conduits, on/off valves, pressure sensors etc is ofthe same type as has been described above for the lifting system. Thehydraulic system 202 for the tilting function comprises a pair of inleton/off valves 277, 278 and a pair of outlet on/off valves 279, 288arranged in the same way as the on/off valves of the hydraulic circuit201 for the lifting function. Further, a pressure sensor 262 is arrangedon the output piston rod side of the cylinder 10 and a pressure sensor263 is arranged on the output piston side of the cylinder 10. The upwardmovement of the load-arm unit 6 may, as an alternative to the means 118for determining a relative angle between the forward vehicle section 2and the load-arm unit 6, be formed by means (see sensor 255 in FIG. 9)for determining a relative angle between the load-arm unit 6 and theimplement 7.

FIGS. 5 and 6 show the same system in two different states when thevehicle reaches a raised portion in the ground. FIG. 5 shows thepositions of the on/off valves during the energy recovery phase and FIG.6 shows the positions of the on/off valves after termination of theenergy recovery phase when the implement 7 and the load-arm unit 6 areautomatically returned to their prior relative position.

The methods for energy recovery from the tilting function work by thesame principle as have been described above for the lifting function;when the detected pressure on either side of the cylinder 10 exceeds theground pressure to a predetermined extent, the outlet on/off valve onthat side is automatically opened so that the energy can be recoveredvia the motor 133. Since the methods have been described in detail forthe lifting function, they will not be repeated for the tiltingfunction.

FIGS. 7 and 8 show a hydraulic system 203 arranged for a steeringoperation of the forward vehicle section 2 relative to the rear vehiclesection 3. The system and energy recovery method for the steeringfunction is very similar to what has been described above for thelifting and tilting function.

The arrangement of conduits, on/off valves, pressure sensors etc is ofthe same type as has been described above for the lifting and tiltingsystem.

The two steering cylinders 4,5 are interconnected by means of twointermediate conduits 240, 241 running crosswise. Thus, the steeringcylinders 4,5 are arranged to simultaneously move in oppositedirections. A first intermediate conduit 240 connects the piston rodside of the first steering cylinder 4 with a piston side of a secondsteering cylinder 5. A second intermediate conduit 241 connects thepiston side of the first steering cylinder 4 with the piston rod side ofthe second steering cylinder 5.

An on/off valve 214, 215, 216, 217 is arranged on each of the fourinput/output conduits of the steering cylinders 4,5. By simultaneouslyopen the on/off valve 214 on the input piston side of the upstreamcylinder 4 and the on/off valve 217 on the output piston side of thedownstream cylinder 5, the vehicle may be turned in a first direction.In the same way, by simultaneously open the on/off valve 215 on theinput piston rod side of the upstream cylinder 4 and the on/off valve216 on the output piston rod side of the downstream cylinder 5, thevehicle may be turned in a second, opposite direction.

The controller 116 is connected to each of the on/off valves 214, 215,216, 217 for electrically controlling them.

The arrangement comprises a first steering means in the form of asteering wheel 221 for operation by an operator, see FIG. 9. An anglesensor 225 of the steering wheel 221 is electrically connected to thecontroller 116. Operation of the steering wheel 221 generates a workfunction signal indicative of a requested steering of the vehicle. Thearrangement further comprises a second steering means in the form of acontrol lever, or joystick, 222 for operation by an operator. Thesteering control lever 222 is electrically connected to the controller116. Operation of the control lever 222 generates a work function signalindicative of a requested steering of the vehicle. The operator of thevehicle may choose which of the two steering means 221, 222 he prefersin a certain situation.

Means 245, 246 are provided for sensing a load pressure subjected to thecylinders 4,5 during operation. Said sensing means is formed byelectrical pressure sensors 245, 246, which generate pressure signals tothe controller 116.

According to the damping and energy recovery method, undesired relativemovements between the forward vehicle section 2 and the rear vehiclesection 3 around a steering joint 140, which arise during advance of thevehicle over uneven ground, are damped and the energy from the relativemovements is recovered.

An undesired relative rotation motion of the forward and rear vehiclesections 2, 3, see arrow 141 in FIG. 7, is detected by pressure sensorsand/or means for determining an angular position between the vehiclesections 2, 3. The piston of a first steering cylinder 4 is then pulledout of the cylinder and the piston of a second steering cylinder 5forced into the cylinder, see arrows 142 and 143 in FIG. 7.

In the same way as has been described above for the alternative liftinghydraulic circuit 201, the power source 103 in the form of a dieselengine for propelling the wheel loader rotatably drives the pump 104.The steering cylinders 4,5 are arranged in fluid connection with thepump 104 via a first conduit. A variable displacement hydraulic motorunit 144 is arranged in fluid connection with the cylinders 4,5 anddownstream the cylinders 4,5 via a second conduit. Said motor unit 144comprises a single motor. The fluid container 120 is arranged downstreamof the motor 144 for collecting hydraulic fluid.

FIGS. 7 and 8 show the same system in two different states. FIG. 7 showsthe positions of the on/off valves during the energy recovery phase andFIG. 8 shows the positions of the on/off valves after termination of theenergy recovery phase when the forward and rear vehicle sections areautomatically returned to their prior relative position.

The methods for energy recovery from the steering function work by thesame principle as have been described above for the lifting function;when the detected pressure on either side of the cylinders 4,5 exceedsthe ground pressure to a predetermined extent, the outlet on/off valveon that side is automatically opened so that the energy can be recoveredvia the motor 144. Since the methods have been described in detail forthe lifting function, they will not be repeated for the steeringfunction.

The variable displacement pump 104 comprises a drive shaft, a rotatablecylinder barrel having multiple piston bores, pistons held against atiltable swashplate, and a valve plate. When the swashplate is tiltedrelative to the longitudinal axis of the drive shaft, the pistonsreciprocate within the piston bores to produce a pumping action anddischarge the pressurized fluid to an outlet port. When the swashplateis positioned at the center and is not tilted, the pistons do notreciprocate and the pump does not produce any discharge pressure.

The variable displacement hydraulic motor 106, 133, 144 comprises adrive shaft, a rotatable cylinder barrel having multiple piston bores,pistons held against a tiltable swashplate, and a valve plate. When theswashplate is tilted relative to the longitudinal axis of the driveshaft, the pistons reciprocate within the piston bores to produce apumping action. The pumping action by the pistons rotates the cylinderbarrel and the drive shaft, thereby providing a motor torque output whenthe fluid pressure at an inlet port is higher than an outlet port. Whenthe swashplate is positioned at the center and is not tilted, thepistons do not reciprocate and the motor does not produce any outputtorque.

Means 106 a, 133 a, 144 a is in operational contact with the swashplateof the associated pump for regulating the displacement. The regulatingmeans 106 a, 133 a, 144 a is electrically controlled by the controller116. The regulating means 106 a, 133 a, 144 a comprises, according toone example, an electrically controlled proportional valve for effectingthe swashplate with pressurized fluid and thereby moving it. Theregulating means 106 a, 133 a, 144 a further comprises an angle sensor,which is arranged to sense the position of the swashplate in order toterminate the movement of the swashplate when the desired angularposition is achieved.

FIG. 9 shows a preferred embodiment of an arrangement for controllingthe wheel loader 1 of FIG. 1. The arrangement comprises the abovedescribed hydraulic systems 201, 202, 203 in somewhat modified form forcontrolling lifting, tilting and steering (turning) of the wheel loader1.

The power source 103 rotationally drives pump 104, which is common forthe three systems 201, 202, 203.

The diesel engine 103 mechanically drives the pump 104 via atransmission 230 and a drive shaft 50. The drive shaft 50 is alsomechanically connected to the motor 144 for the steering function. Thus,the pump 104 and the motor 144 rotates at the same speed duringoperation.

The hydraulic circuit forms a load sensing system. The load sensinghydraulic system is characterized by that the operating condition of theload is sensed and that the output pressure of the pump 104 iscontrolled so that it exceeds the load pressure existing in thecylinders by a predetermined differential.

Further, an electrically controlled pressure reducing valve 247 isarranged in connection to the pump 104 for regulating the outputpressure of the pump. The pressure reducing valve 247 is arranged on aside conduit between a first conduit 206 from the pump 104 to thesteering cylinder 4,5 and the displacement control means of the pump 104for regulating a fluid connection between the first conduit and thepump. In other words, the pressure reducing valve 247 is adapted to senda hydraulic LS signal to the pump 104 depending on a signal from thecontroller 116. Thus, the signal from the controller may be dependent orindependent of the pressure level sensed by the pressure sensors 245,246.

Further, in some load cases, there is a requirement to aid to after-fillthe cylinders when the pump 104 cannot supply the desired fluid flow. Atwo position backup valve 260 is arranged downstream of the motors 106,133, 144. Below follows a description of the after-filling with regardto the lifting cylinder, but it is equally applicable for the steeringand tilting cylinders.

Said non-return valve 125, 127 is arranged on the outlet conduit 123,126 connected to the piston rod side and the piston side, respectively,of the cylinder. These outlet conduits 123, 126 merge to a commonconduit 265 connected to the motor 106 downstream of the motor 106,bypassing the backup valve 260. A pilot pressure conduit 259 isconnected to the common conduit 265 and to a pilot pressure side of thebackup valve 260 for acting on the backup valve with a pilot pressure.In this way, the backup valve may block the fluid connection from themotor 106 to the tank 209 and the fluid will therefore flow back to thecylinder via a conduit 267 bypassing the backup valve 260, via thecommon conduit 265 and the outlet conduit 123, 126.

The backup valve 260 is arranged to be closed when there is a need toafter-fill the cylinders and be open when no after-fill is needed. A rod268 is connected to one side of the backup valve 260 opposite the pilotpressure side. The rod 268 has two grooves at a distance from eachother, defining the two positions of the backup valve 260. A springloaded ball 269 is adapted to be received in one of said grooves at atime. Further, the backup valve 260 is spring loaded via a spring 270.

An accumulator 266 is in fluid connection with the common conduit 265,which extends between the motor 106 and the outlet side of thedownstream cylinder 9. The accumulator 266 is arranged in such a waythat the backup valve 260 will not be moved too frequently. Thus, itextends the life of the backup valve. When the accumulator 266 ischarged to a certain level, the backup valve 260 will open completelyand there will be no pressure drop over the valve. When the pressure ofthe accumulator 266 falls to a certain level, the backup valve willclose again and the accumulator 266 will be recharged. When there is noneed to after-fill the cylinder, the accumulator will provide asufficient pressure in order to keep the backup valve in the openposition and thereby not generate any pressure drop. The backup valve260 is required to have a certain hysteresis. The backup valve 260 isdesigned to close at a low pressure level (for example 4 bar) and openat a higher pressure level (for example 8 bar).

The function of backup valve 260 system described above is not onlyapplicable when the pump cannot supply the desired fluid flow to thecylinder. It is also applicable for example when the load arm unit 6 islowered or when the bucket 7 is emptied and the movement is performedtotally by the action of the gravity force. The inlet side of thecylinder may in this case be closed and the pump may be used for otherpurposes.

Means 273 is arranged to sense a relative angle between the forwardvehicle section 2 and the rear vehicle section 3. The sensor 273 iselectrically coupled to the controller 116. Thus, the controller 116receives information about the relative position of the two vehiclesections.

The recovered energy may be used by the engine 103 to drive othersystems, like a vehicle driveline 287 and service brakes 285, andcomponents like fans 286, generators etc, via a branch line 284. Asecond pump 271 is arranged for supplying the components 285, 286 withpressurized fluid and is rotationally driven by the engine 204 via thetransmission 230.

The lift cylinders 8,9 are arranged to simultaneously move in the samedirection. The lift cylinders 8,9 are interconnected by means of twointermediate conduits 280, 281. A first intermediate conduit 280connects the piston rod sides of the cylinders 8,9 and a secondintermediate conduit 281 connects the piston sides of the cylinders 8,9.

The system comprises a tilting control means 224, in the form of acontrol lever, for operation by an operator. The tilting control means224 is electrically connected to the controller 116. Operation of thetilting control means 224 generates a work function signal indicative ofa requested tilting of the bucket 7.

A coupling means 296 is arranged between the engine 204 and the motors106, 133 for the lifting and tilting functions for disconnecting themotors from a driving connection with the engine. More specifically, thecoupling means 296 is arranged on a common drive shaft 283 between themotors 106, 133 and the transmission 230. The coupling means 296 isformed by a hydraulic disc clutch or a freewheel.

A generator 297 is rotationally connected to the engine 103. In theshown example in FIG. 9, the generator 297 is connected on an outputshaft 233 from the engine 103, between the engine 103 and thetransmission 230. The recovered energy from the motor (s) 106, 133, 144may be stored in the generator 297. As an alternative, a battery (notshown) is connected to the generator 297. The battery may in turn beconnected to a further energy consumer. The generator 297 may further beused as a motor and regenerate energy from the battery.

The wheels of the wheel loader 1 are driven by the half shafts 12, 13,see FIG. 1, which in turn are driven by the engine 103 via the drivelinein a per se known way. A converter 287 in the driveline is indicated inFIG. 3. The converter 287 is driven by the engine 103 via thetransmission 230. Any recovered energy in the hydraulic motors 106, 133,144 may be used for propelling the vehicle via the converter 287.

The work vehicle may have a hydrostatic transmission. In such a case,the recovered energy may also be used by the engine 103 to drive pumpsor other components in the hydrostatic transmission.

Further, thanks to the invention, conditions are created for integrationof pump functions of different systems in the vehicle.

According to a first example, the vehicle is equipped with a hydrostatictransmission. The hydrostatic transmission may comprise two pumps. Thesepumps may partly be used for work functions like lift, tilt andauxiliary functions. These work functions do not need high flows whenthe vehicle is driven with high speed, which means that the pumps can beused for propelling the vehicle. Instead, said work functions requirelarger flows at lower vehicle speeds, when the hydrostatic transmissiondoes not require large flows. Thus, the pump flow requirements of saidwork functions and the hydrostatic transmission complement each other.In the case that the hydrostatic transmission only has one pump, it mayalso be used for both the hydrostatic transmission and to said workfunctions. In the latter case, each system needs to be able to managethe maximum pressure level of the other system.

The controller 116 comprises a memory, which in turn comprises acomputer program with computer program segments, or a program code, forimplementing the control method when the program is run. This computerprogram can be transmitted to the controller in various ways via atransmission signal, for example by downloading from another computer,via wire and/or wirelessly, or by installation in a memory circuit. Inparticular, the transmission signal can be transmitted via the Internet.

The invention also relates to a computer program product comprisingcomputer program segments stored on a computer-readable means forimplementing the measurement method when the program is run. Thecomputer program product can consist of comprise, for example, adiskette or a CD.

Although the power source is arranged for propelling the vehicleaccording to the preferred embodiments described above, the term “powersource” is not limited to a unit for propelling the vehicle. The term“power source” should be interpreted in a broad sense to also compriseany energy consuming, energy absorbing or energy storing source. Such anenergy storing source may be conFIG.d to store energy hydraulically, forexample with an accumulator, electrically, for example by means of agenerator, or mechanically, for example by means of an apparatus forrotating masses. Further, the power source may be an electric engine ora hydraulic pump.

The invention is not in any way limited to the above describedembodiments, instead a number of alternatives and modifications arepossible without departing from the scope of the following claims.

According to an alternative to using only one motor 106, 133, 144 foreach work function, the term motor unit comprises a plurality of motors.The plurality of motors in a single motor unit may be arranged in serieson a common drive shaft. The plurality of motors in a single motor unitare further arranged in parallel with respect to a fluid connection tothe associated actuator so that at least one of the motors in the motorunit may be disconnected from fluid connection with the associatedactuator.

According to a further alternative, several work functions may share onesingle hydraulic motor unit. Each such work function is then arranged tobe connectable to the single motor unit via a respective valve unitarranged between the hydraulic cylinder and the motor unit. Preferably,the valve unit is adapted to either connect the hydraulic cylinder ofthe work function in question to the single motor unit or directly totank.

One of said work functions could be to rotate an upper section of thevehicle in relation to a lower section of the vehicle. This is acommonly used arrangement for excavators, where the upper sectioncomprises a cab and the lower section comprises ground engaging members,like tracks or wheels. The actuator is in this case formed by ahydraulic motor.

As an alternative to the arrangement of the hydraulic motors 106, 133 onthe common drive shaft 283, see FIG. 9, the two motors may be arrangedon different drive shafts.

According to one alternative of the above described embodiment, in whicha common pump is used for all work functions, one pump may be used foreach work function.

According to an alternative use of the method described above, it may beused for recovering energy from external loads or chocks applied to thesystem. Such a chock may arise when the vehicle is used for bucketoperation and the bucket is brought into contact with a stone or similarhard material when the vehicle is forwarded into the material to behandled. A further example of generation of such a chock may ariseduring timber handling if timber logs fall down on the equipment. Suchoccurrence would lead to a high pressure in the lift cylinder. When sucha high pressure is detected, the chock energy may be recovered via themotor unit in the way described above. The position of the hydrauliccylinder may thereafter be adjusted by supplying pressurized hydraulicfluid from the pump.

The invention claimed is:
 1. A method for damping relative movementsoccurring between a first and a second part of a work vehicle duringadvance of the vehicle, which parts are interconnected by means of atleast one hydraulic actuator, comprising advancing the vehicle byrolling the vehicle by way of ground-engaging elements, repeatedlydetecting a hydraulic fluid pressure associated with the actuatorresulting from rolling the vehicle, determining whether a dampingcondition is present based on a predetermined change in values of thehydraulic fluid pressure that is detected, if the determined dampingcondition is present, controlling a displacement of a variabledisplacement hydraulic motor arranged downstream of the actuator byinitiating an energy recovery phase during which flow communicationbetween a chamber of the actuator and the motor is opened and hydraulicfluid is transmitted from the actuator to the motor so that energy fromthe hydraulic fluid is recovered by being converted to a rotationalenergy in the motor, transmitting the recovered energy from the motor toa power source, and after termination of the energy recovery phase,automatically moving the first and second parts relative to one anotherto a relative position existing prior to determining whether the dampingcondition is present.
 2. A method according to claim 1, comprisingdetecting an angle between the first and second parts, and determiningwhether the damping condition is present based on a value of the anglethat is detected.
 3. A method according to claim 2, further comprisingrepeatedly detecting the angle and determining whether the dampingcondition is present based on a predetermined change in values of theangle that is detected.
 4. A method according to claim 1, comprisingrepeatedly detecting an angle between the first and second parts, anddetermining whether the damping condition is present based on apredetermined change in values of the angle that is detected, anddetermining the damping condition as a function of a relationship in thechange in the values of the angle that is detected and the change in thevalues of the hydraulic fluid pressure that is detected.
 5. A methodaccording to claim 1, wherein the first part is formed by an arm of aload-arm unit and the second part is formed by a vehicle sectioncomprising a vehicle frame part.
 6. A method according to claim 1,wherein the first part is formed by an arm of a load-arm unit and thesecond part is formed by an implement.
 7. A method according to claim 1,wherein the first part is formed by a first vehicle section comprising afirst vehicle frame part and the second part is formed by a secondvehicle section.
 8. A method according to claim 1, further comprisingdetermining a termination of an energy recovery phase, the energyrecovery phase having been initiated by the determination of the dampingcondition, by repeatedly detecting the at least one operation parameter.9. A method according to claim 1, further comprising initiating theenergy recovery phase by the determination of the damping condition. 10.A method according to claim 9, wherein the two parts are moved relativeto one another by means of the actuator.
 11. A method according to claim9, further comprising the step of closing a fluid connection from theactuator to the motor after termination of the recovery phase.
 12. Acomputer comprising computer program segments for implementing themethod as claimed in claim
 1. 13. A computer program product comprisingcomputer program segments stored on a non-transitory computer-readablemedium for implementing the method as claimed in claim 1.